It is commonplace in the field of exploration for oil, gas and other minerals to use seismic techniques, in which a burst of acoustic energy is imparted to the earth or sea at a "shot point". The acoustic energy travels through the earth, including the ocean, and is reflected at the interfaces between rock layers of varying density to be returned back upwardly to the surface of the earth. Its arrival can be detected by an acoustic microphone, typically referred to as a geophone in earth-based processing and as a hydrophone in water-based prospecting.
Due to the spherical dispersion of the acoustic wave as it travels through the earth, the further the wave travels into the earth before it is reflected back upwardly, the more faintly it is detected upon its eventual arrival at a detector. Additionally, the presence of strongly reflecting rock layers interspersed with more weakly reflecting beds leads to substantial differences in the strength of the detected signals. For these reasons, the art has conventionally applied automatic gain control to the various portions of the seismic signal so that they can be more readily compared to one another, such that distinct events in the fainter portion of the data are not overlooked in comparison to the less attenuated portions. Typically what is done is that a "window" is established, this being a number of samples, e.g. 800 milliseconds long. The data is divided into overlapping windows of this length and the average values or root mean square values of the absolute values of the data in each window are calculated. These average absolute values are used to generate automatic gain control factors, for each window, which are then compared to one another, and can be applied to all of the samples within a given window.
The difficulty with this approach is that high amplitude events within a given window greatly exaggerate the amount of correction to be applied to the other samples in that window, such that a "shadow" effect is observed in the automatic gain control processed data. Relatively faint events within the shadow are thus obscured.